Virtual surround-sound systems take advantage of the basic properties of speakers, sound waves and hearing. A speaker is essentially a device that changes electrical impulses into sound. It does this using a diaphragm -- a cone that rapidly moves back and forth, pushing against and pulling away from the air next to it. When the diaphragm moves outward, it creates a compression, or area of high pressure, in the air. When it moves back, it creates a rarefaction, or area of lower pressure. You can learn more about the details in How Speakers Work.
Compressions and rarefactions are the result of the movement of air particles. When the particles push against each other, they create an area of higher pressure. These particles also press against the molecules next to them. When the particles move apart, they create an area of lower pressure while pulling away from the neighboring particles. In this manner, the compressions and rarefactions travel through the air as a longitudinal wave.
When this wave of high- and low-pressure areas reaches your ear, several things happen that allow you to perceive it as sound. The wave reflects off of the pinna, or external cone, of your ear. This part of your ear is also known as the auricle. The sound also travels into your ear canal, where it physically moves your tympanic membrane, or eardrum. This sets off a chain reaction involving many tiny structures inside your ear. Eventually, the vibrations from the wave of pressure reach your cochlear nerve, which carries them to the brain (brain.htm) as nerve impulses. Your brain interprets these impulses as sound. How Hearing Works (hearing.htm) has lots more information about your ear's internal structures and what it takes to perceive sound.
Your brain's interpretation process allows you to understand the sound's meaning. If the sound is a series of spoken words, you can put them together into an understandable sentence. If the sound is a song, you can interpret the words, experience the tone and rhythm, and decide whether you like what you hear. You can also remember whether you've heard the same song or similar songs before.
In addition to allowing you to interpret the sound, your brain also uses lots of aural cues to help you figure out where it came from. This isn't always something you think about or are even consciously aware of. But being able to locate the source of a sound is an important skill. This ability helps animals locate food, avoid predators and find others of their species. Being able to tell where a sound came from also helps you decide whether someone is following you and whether a knock outside is at your door or your neighbor's.
These cues and the physical properties of sound waves are central to virtual surround sound. We'll look at them in more detail next.